Just because you did not receive an email does not mean that it was not filtered along the way.
It depends on how your MX records are configured and what anti-spam filtering you have in place.
Email is NOT a guaranteed method of communication.
If you didn't receive emails when your service was down, then it is down to the sending serer to re-send you the message. If they are set to timeout after 24 hours of trying, then there is nothing you can do.
Also, even if you request a delivery receipt, there is no guarantee of you getting it, or you may also not get an NDR messaage - as not all systems are configured to provide them.
Check your MX records on www.mxtoolbox.com and see how many you have configured. If you only have one and it points to your fixed IP Address, then all mail is sent directly to you. If there are others, then it depends on the MX record priority. If yours is lower than the others, then your mail server is the preferred one and the others are there as fallback, but some servers ignore the MX record priority and send to the one that they have in cache. This could mean that your lost mail goes directly to the secondary mail server and if you don't collect it, or they don't forward it, you will never receive it.
